How critical is definition of myocardial region of interest for determination of heart-mediastinum ratio on planar 123I-mIBG myocardial imaging?

Objectives In planar 123I-mIBG myocardial imaging, definition of the heart (H) region of interest (ROI) is central to quantitation of global cardiac uptake. The present study was undertaken to evaluate the impact of changes in H ROI size on these quantitative results.

Methods An experienced nuclear medicine technologist defined irregular whole H and square upper mediastinum (M) ROIs on 535 planar (128x128 matrix) 123I-mIBG images (423 late (4 hours post-injection), 112 early (15 minutes post-injection)) acquired on 423 heart failure (HF) and control subjects in 4 clinical trials. Using the visually defined H ROI as the reference, an automated program was used to draw 2 additional ROIs, one larger (+1 pixels) and 1 smaller (-1 pixels). The stated numbers represent the spacing interval between the epicardial boundary pixels for the new ROIs relative to those for the reference ROI. The counts/pixel (cpp) in each H ROI were calculated and compared to that in the reference ROI.

Results The mean size of the reference H ROI was 568 pixels (SD ±359), and the mean cpp was 602 (±421). A change of +1 pixel and -1 pixel in the boundary of the H ROI resulted in mean changes of +19.2% and -17.7% in ROI size. The corresponding changes in the mean cpp were -1.0% and + 0.7% respectively. For nominal H/M ratios of 1.20 and 1.60 based upon the reference ROI, these would represent H/M ranges of 1.19-1.21 and 1.58-1.61 respectively.

Conclusions For a mean 37% change in the size of the H ROI, the mean change in cpp is <2%, which would result in a clinically insignificant change in the calculated H/M ratio. As such, the H/M ratio determined using a whole-H ROI should be robust, with differences between processors’ assessments of the exact position of the ROI being of minimal clinical consequence.

Research Support GE Healthcar